Transmission cables used to transport electricity or communication signals are made up of one or more conductors surrounded by insulation material. Any imperfections in the insulation may lead to short circuiting and premature failure of the cable. For this reason, cables are typically subjected to spark testing as part of the manufacturing process. A popular method of spark testing includes introducing the insulated cable to a high voltage field while the one or more conductors inside the insulation remain grounded. Any imperfections in the insulation should result in a spark created between a conductor within the cable and the closest electrode that is providing the voltage. Common spark testing equipment includes bead testers. A bead tester includes a box having a large number of bead chains, or bead electrodes, hanging from the top of the box. The cable undergoing the spark testing will be pulled through the chains hanging from the top of the box. Voltage is then applied to the bead electrodes in an attempt to induce a spark when an imperfection in the insulation is located.
When the cable is physically contacting the source of the voltage, the bead electrodes, the conductor within the cable is as close as possible to the electrodes, assuring the highest percentage of success in locating imperfections. An electrode contacting the surface of the cable is more likely to induce a short circuit to the conductor through an imperfection in the intervening insulation than would an electrode that is an inch away from the cable, since the path from the electrode to the conductor is shorter when the electrode is contacting the cable. By utilizing a large number of bead chains within the box, there is a relatively high likelihood that the top and sides of the cable will come into contact with one or more bead electrodes containing the test voltage.
However, because gravity forces the bead chains to hang vertically straight down, the underside of the cable will never be in contact with the bead electrodes. Although, in many cases, the voltage from the bead electrodes closest to the underside of the cable is strong enough to induce a short circuit to the cable conductor when an imperfection on the underside of the cable is present. However, because the underside of the cable does not contact the bead electrodes, it is likely that some imperfections on the underside of the cable will be missed.